The present invention relates in general to magnetic field-based communication and proximity detection systems, and is particularly directed to a tagged object information storage and retrieval system, that employs a spread spectrum modulated magnetic field for identifying each of a plurality of tagged objects to which spread spectrum modulation magnetic field transponders are attached. When interrogated by a tag reader, transponders embedded in one or more tags generate spread spectrum modulated magnetic fields that are correlated with a reference spreading sequence in a tag reader signal processor to both detect and identify each responding tag.
The identification of objects, such as packages, retail sales items, shipping containers, and the like, is often accomplished by affixing to the object an information storage medium such as a xe2x80x98bar codexe2x80x99 label encoded with previously assembled and stored information that fully describes the object. In order to read a bar code label, the labelled object must be oriented so that the bar code may be xe2x80x98viewedxe2x80x99 by an optical (e.g. laser) scanner, either as a stationary device or a hand held unit. A familiar example of the use of such a scanner is a check-out line of a retail sales establishment, where a clerk orients each item with the bar code label face down, and then passes the item across an optical scanner viewing window. As the object is passed over the window, the bar code is read by the scanner, which outputs data to an adjacent point-of-sales terminal, where the transaction is processed. If the object is not properly oriented so as to allow the scanner to read the label, the sales clerk will not hear an audible tone confirming success of the scan, and will repeat the process.
A similar technique is employed in the transportation industry, where items such as baggage being unloaded from a vehicle are placed upon a conveyor belt in a prescribed orientation, so that they may be viewed by the scanner. If the object has not been properly placed on the conveyor, the object will require further handling to ensure that the bar code label can be read by a downstream scanner. This need to physically orient encoded label-containing items relative to an optical scanner is both labor-intensive and time-consuming, and constitutes an unwanted expense.
Non-limiting examples of proposals to remedy this viewing orientation problem include a variety of what are generally referred to as xe2x80x9cRFIDxe2x80x9d (radio frequency identification) systems. Non-limiting examples of such RFID systems which employ magnetic fields are described in the U.S. Patents to Bickley et al, U.S. Pat. No. 5,430,441; Watanabe et al, U.S. Pat. No. 5,478,991; and Brooks et al, U.S. Pat. No. 5,485,154. In each of these schemes, a tag transponder is powered from a continuously generated magnetic field. In the Bickley et al and Watanabe et al approaches the transponder responds by modulating that same continuously transmitted electromagnetic field. In the Brooks et al scheme, the tag responds by generating a signal at a frequency different from the exciting frequency to avoid interference. An obvious drawback of these patented approaches is the fact that they continuously consume power. Indeed, in the Brooks et al scheme, the amount of excitation energy required is so large, that it requires the use of a shielded tunnel to prevent electromagnetic contamination of the surrounding environment.
In accordance with the present invention, a tagged object information storage and retrieval system employs a spread spectrum modulated magnetic field for locating and identifying a tagged object. Not only does spread-spectrum signaling provide a substantial degree of immunity against interferers, but effectively immunizes each transponder against its own signal, because of the extremely low probability of that signal being precisely time-aligned with those of other receivers. As a consequence, multiple transponders emitting the same spread magnetic field in response to an interrogation stimulus from a tag reader may be independently detected.
The transponder circuitry of a respective tag may be affixed to an object by means of adhesively backed strip of protective material, that contains a thin coil. The coil is coupled to a power bus, through which electrical power is extracted, rectified and stored from an intercepted excitation magnetic field and supplied to the transponder circuitry, when the coil is intercepted by a time varying magnetic field, such as a low duty cycle repetitive excitation pulse generated by the tag reader. The transponder coil is also further coupled to a reply energy storage capacitor, which stores extracted energy to be controllably applied to the coil by way of a modulated FET switch to generate a spread spectrum-modulated magnetic field that contains information stored in the transponder""s non-volatile memory during a response mode of operation of the transponder.
The coil output is further coupled to a pulse counter, which controls the mode of operation of a transponder state machine, in accordance with the number of half cycles in each excitation magnetic field burst from the reader. When enabled during a xe2x80x98responsexe2x80x99 mode of operation, the state machine accesses data stored in memory in preparation for transmission in a spreading signal-based reply message. The state machine gates the FET switch with the spreading signal, as exclusive-OR modulated by the data, so as to cause a baseband spread magnetic field to be emitted from the transponder coil.
The reader includes a pulse generator, which is coupled to a multiple excitation and interrogate/reader coil arrangement, in which receiver coils are arranged close to the excitation coil and are connected in opposite polarity. This arrangement produces an electrical output representative of a differential combination of responses from all impinging magnetic fields. As a result, energy received from the excitation coil and relatively distant sources is canceled, while providing a useful net output for a field generated by a nearby (object-tagged) transponder. The output from the reader coils is clamped, filtered and digitized for application to a digital correlator, which correlates the response emitted from the interrogated tag""s transponder with a replica of the spreading signal.
The high probability that no two responses from tag transponders will be time-aligned or have exactly the same clock frequency (due to variations in their internal clocks) will tend to decorrelate their responses, even if they commense at the same time. This allows matched filter processing of the responses, for the purpose of individually segregating each response, even though each transponder employs the same PN sequence to spread its own signal.